1. Field of the Invention
The present invention relates to zirconia and/or hafnia-containing ceramics, particularly ceramics having a high electric resistance and ceramics having a high mechanical strength.
2. Description of the Prior Art
Heretofore, a variety of studies have been made with respect to ceramics consisting mainly of zirconia from both views of the functional materials and the constructive materials as solid electrolytes, parts having mechanical high strength and the like. In particular, when zirconia ceramics are used as a solid electrolyte, the internal resistance of a cell is preferred to be low in order to take out the electromotive force as an oxygen concentration cell and such ceramics that an electric resistance is as low as possible at a high temperature range of higher than about 350.degree. C., have been used. In these ceramics, the high mechanical strength has been demanded in order to improve the vibration resistance and the thermal shock resistance.
Embodiments of such zirconia ceramics are disclosed in U.S. Pat. No. 4,266,979, U.S. Pat. No. 4,360,598 and Japanese Patent Laid-Open Application No. 58-55,373.
The reason why zirconia ceramics can be used as a solid electrolyte is due to oxygen ion vacancy caused by addition of a stabilizer such as Y.sub.2 O.sub.3, CaO and the like to ZrO.sub.2 as well known. That is, when the position Zr.sup.4+ of positive tetravalent ion is substituted with Y.sup.3+ or Ca.sup.2+ of positive trivalent or divalent ion, the positive ion valence number per crystal lattice is reduced, so that the number of oxygen ion O.sup.2- having negative ion is reduced for maintaining the electric neutrality and the movement of oxygen ion becomes feasible due to oxygen ion vacancy caused therefrom.
The concentration cell constructed with such a theory is reversible and if there is difference of the oxygen concentration between both ends of the solid electrolyte, the electromotive force is caused by the well known Nernst's equation ##EQU1## and reversely, when a direct current voltage is applied to both ends of the solid electrolyte to flow current, oxygen ion moves from negative direction toward positive direction and oxygen can be moved from one side of the solid electrolyte to another side. This is well known as oxygen pump.
When a direct current voltage is applied to conventional zirconia ceramics to flow current, if the applied voltage is lower than 1 V, the movement of the oxygen ion occurs moderately, so that an oxygen pump is obtained but when the applied voltage is higher and becomes about 10 V, the movement of oxygen ion can not flow to the direct current and polarization occurs in the interior of zirconia ceramics and oxygen ion in the negative side is deficient and zirconia ceramics is broken down.
Namely, the prior zirconia ceramics have been very weak against the application of the direct current voltage.
When such zirconia ceramics are used as a solid electrolyte for an oxygen sensor, the drawbacks appear in the following case.
In particular, when zirconia ceramics are used as an oxygen sensor for automotive exhaust gas, in order to operate the sensor at the exhaust gas temperature of as low as about 350.degree. C., the portion constructing the concentration cell is heated with a heater. This structure is shown, for example, in U.S. Pat. No. 4,334,974. In the oxygen sensor having such a structure, the heater is heated at a direct current voltage of about 12-14 V which is battery voltage and therefore an insulating layer must be interposed between the heater and the concentration cell in order that the voltage for heating the heater does not influence upon the electromotive force of the concentration cell. If a conventional zirconia ceramics is used as this insulating layer, the zirconia ceramics is not only broken by the direct current voltage of 12-14 V applied to the heater, but also the voltage applied to the heater influences upon the electromotive force of the concentration cell due to zirconia of which the electric resistance becomes lower at a high temperature. When alumina ceramics which is a high resistor as an insulating layer, is used, the insulating property is good but said ceramics are different from zirconia ceramics constructing the concentration cell in the thermal expansion coefficient, so that in the use when the heat cycle between a high temperature and a low temperature is vigorous, alumina ceramics is exfoliated from the concentration cell composed of zirconia ceramics. Furthermore, such an oxygen sensor has a drawback that in the production, when zirconia and alumina which are different in the firing shrinkage percent, are co-fired, zirconia and alumina are exfoliated upon firing and cooling.